The present invention relates to a method of producing a ceramic honeycomb body with low CTE (&lt;7.times.10.sup.-7.degree. C..sup.-1 from 25.degree.-800.degree. C.) which imparts high thermal shock resistance to the body. The method centers on the use of a finely dispersible, high surface area alumina-yielding source as a raw material in the batch mixture. The invention is especially useful for thin walled honeycomb substrates since it allows for the use of a fine talc source while maintaining a high degree of thermal shock resistance in the fired body.
Many different approaches to the use of alumina and talc in cordierite ceramic product formulations are known. British Patent Specification Number 1,518,475 and U.S. Pat. No. 4,280,845, for example, demonstrated that a cordierite ceramic having a satisfactory thermal expansion coefficient is obtained when the average particle size of the magnesium-containing raw material (specifically talc) is within the range of 5-150 micrometers, preferably 21-100 micrometers, and more preferably, 26-80 micrometers. It is shown that when the average particle size of the magnesium-containing raw material is outside this range, the thermal expansion coefficient over the range of 25-1000.degree. C. is greater than 16.times.10.sup.-7.degree. C..sup.-1, resulting in poor thermal shock resistance.
U.S. Pat. No. 4,434,117 describes a method of producing a cordierite honeycomb body with a low coefficient of thermal expansion where the average talc particle size is as low as 2.8 micrometers. However, this method requires the "pre-firing" or calcination of the talc particles. Without this pre-firing step, it was found that the thermal expansion coefficient was unacceptably high when the average talc particle size was less than about 20 micrometers.
U.S. Pat. No. 4,772,580 and European Patent Number 0,232,621 both describe a method to obtain a low expansion cordierite honeycomb body using very fine talc particles (&lt;7 micrometers). The method relies on the use of very fine clay in conjunction with the fine talc. It is specified that the average particle size of the clay particles can be no more than 2 micrometers, and the ratio of clay particle size to talc particle size can be no more than 1/3. The resulting ceramic product has a low level of porosity (&lt;30%). Although low levels of porosity result in higher strength, it is known that high levels of porosity are most desirable for excellent coatability with a high surface area washcoat and catalyst.
In U.S. Pat. Nos. 4,772,580, 4,869,944, 5,030,398, 5,409,870 and European Patent Number 0,232,621, at least one material selected from the group consisting of alumina particles and aluminum hydroxide particles having an average particle size not greater than 2 micrometers is added to the batch for the low coefficient of thermal expansion. The aluminum hydroxide referred to in these patents is aluminum tri-hydrate, showing a 34% ignition weight loss on firing. Aluminum tri-hydrate typically has a relatively low surface area (&lt;10 m.sup.2 /g), compared to boehmite (alpha aluminum monohydrate) which typically has a very high surface area (&gt;50 m.sup.2 /g).
In Japanese published patent publication No. 256965/86, a cordierite honeycomb with a high thermal shock resistance is reportedly obtained by using .alpha.-alumina as a batch constituent, where the particle size distribution of the .alpha.-alumina is adjusted so that the particles with diameter less than 3 micrometers are not more than 17% of the distribution by weight, and the median diameter is between 4 and 17 micrometers. Fine .alpha.-alumina and intermediate phases such as .chi.-, .kappa.-, .gamma.- , .delta.- , .theta.-alumina etc. are considered highly reactive with respect to magnesium-containing raw material at a relatively low temperature (&lt;1300.degree. C.), being thought to hinder the main reaction with the talc and kaolin for producing the low thermal expansion cordierite.
In U.S. Pat. No. 5,030,592, a process for the manufacture of high density cordierite by the sol-gel process is described. In this patent, an alumina sol is used, along with magnesium and silicon oxide sols, to form a gel which is then dried and fired to form a cordierite body. Boehmite is used as one alumina source, although effects of its substitution for other aluminas in sol-based systems was not noted.
In U.S. Pat. No. 5,258,150, the use of boehmite is included under the category of aluminum oxide-yielding components (aluminum oxy-hydrate) in a method for the fabrication of a low thermal expansion, high porosity body. However, no recognition of the beneficial effects of aluminum mono-hydrate or other finely dispersible aluminum-yielding source with very high surface area is expressed.
In U.S. Pat. No. 5,332,703, a method for producing a low porosity, high strength cordierite body with low thermal expansion coefficient is described. In this patent, a composition consisting of a mineral component (talc, clay, alumina), is mixed with a chemical component (powdered oxides, hydroxides, and hydrous oxides of magnesium, silicon, and aluminum). However, no trends linking increases in the chemical component with decreases in thermal expansion were observed.